The field of the invention is direct gas-fired heating and ventilation systems for industrial and commercial buildings, and particularly to systems which use variable amounts of fresh air.
Direct gas-fired heating and ventilating systems provide heated make-up air to buildings by drawing fresh air in from outside the building. Such make-up air is necessitated by the loss of heated inside air through open doors, chimney flues and exhaust fans. To prevent infiltration of cold outside air, it is common practice to provide more make-up air than is lost so that a slight positive pressure is maintained inside the building. The operation of the heating and ventilation system is controlled to maintain a selected inside air pressure and temperature.
There are two basic types of direct gas-fired heating and ventilating systems in use. One type includes a fresh air intake which communicates with a chamber containing a burner that heats the air as it is drawn or blown over the flame by a blower. The heated air is exhausted into the building. In some systems the blower speed is controlled to vary the amount of heated make-up air generated, and in other systems the fan is operated at a constant speed and is turned on and off as needed. Example systems of this type are disclosed in U.S. Pat. Nos. 4,325,352; 3,591,150 and 4,278,423.
The second basic type of system recirculates inside air and mixes it with the heated fresh air before delivering it to the inside space. These systems include a recirculating air intake which draws air from inside the building and mixes it with the heated fresh air. Dampers control the relative amounts of each air stream. Examples of this type of direct gas-fired heating and ventilating system are disclosed in U.S. Pat. Nos. 3,417,977; 4,674,475; 4,573,912; 4,429,679 and 4,917,074.
For proper combustion it is imperative that the air stream over the direct gas-fired burner be maintained at a constant velocity. This requires that the pressure drop across the burner be maintained at a relatively constant level. In the above-cited patents this is achieved by controlling dampers that supply air to the burner or dampers that shunt air around the burner. In all cases, the dampers are either controlled by complex interlocking mechanisms or are separately controlled by electronic controllers such as those disclosed in U.S. Pat. Nos. 4,829,447; 5,597,354 and 5,257,958.